NCP1403_05 Datasheet, PDF (9/18 Pages) ON Semiconductor – 15 V/50 mA PFM Step−Up DC−DC Converter

NCP1403
DETAILED OPERATING DESCRIPTION
Operation
The NCP1403 is monolithic DC−DC switching converter
optimized for single Lithium or two cells AA/AAA size
batteries powered portable products.
The NCP1403 device consists of startup circuit, chip
enable circuit, PFM comparator, voltage reference, PFM
on/off timing control circuit, driver, current limit circuit, and
open−drain MOSFET switch. The device operating current
is typically 130 mA, and can be further reduced to about 0.3
mA when the chip is disabled (V_CE < 0.3 V).
The operation of NCP1403 can be best understood by
referring to the block diagram and typical application circuit
1 in Figures 3 and 1. The PFM comparator monitors the
output voltage via the external feedback resistor divider by
comparing the feedback voltage with the reference voltage.
When the feedback voltage is lower than the reference
voltage, the PFM control and driver circuit turns on the
N−Channel MOSFET switch and the current ramps up in the
inductor. The switch will remain on for the maximum
on−time, 6.0 ms, or until the current limit is reached,
whichever occurs first. The MOSFET switch is then turned
off and energy stored in the inductor will be discharged to the
output capacitor and load through the Schottky diode. The
MOSFET switch will be turned off for at least the minimum
off−time, 1.3 ms, and will remain off if the feedback voltage
is higher than the reference voltage and output capacitor will
be discharged to sustain the output current, until the
feedback voltage is again lower than reference voltage. This
switching cycle is then repeated to attain voltage regulation.
Soft Start
There is a soft start circuit in NCP1403. When power is
applied to the device, the soft start circuit pumps up the
output voltage to approximately 1.5 V at a fixed duty cycle,
the level at which the converter can operate normally. With
the soft start circuit, the output voltage overshoot is
minimized and the startup capability with heavy loads is also
improved.
ON/OFF Timing Control
The maximum on−time is typically 6.0 ms, whereas, the
minimum off−time is typically 1.3 ms. Owing to the current
limit circuit, the on−time can be shorter. The switching
frequency can be up to 300 kHz.
Voltage Reference and Output Voltage
The internal voltage reference is trimmed to 0.8 V at an
accuracy of ±5.0%. The voltage reference is connected to the
non−inverting input of the PFM comparator and the
inverting input of the PFM comparator is connected to the
FB pin. The output voltage can be set by connected an
external resistor voltage divider from the V_OUT to the
FB pin. With the internal 16 V MOSFET switch, the output
voltage can be set between V_IN to 15 V.
LX Limit
The LX Limit is a current limit feature which is achieved
by monitoring the voltage at the LX pin during the MOSFET
switch turn−on period. When the switch is turned on, current
ramps up in the inductor, and the voltage at the LX pin will
increase according to the Ohm’s Law due to the On−state
resistance of the MOSFET. When the VLX is greater than
0.75 V, the switch will be turned off. With the current limit
circuit, saturation of inductor is prevented and output
voltage overshoot during startup can also be minimized.
N−Channel MOSFET Switch
The NCP1403 is built−in with a 16 V open drain
N−Channel MOSFET switch which allows high output
voltage up to 15 V to be generated from simple step−up
topology.
Enable / Disable Operation
The NCP1403 offers IC shut−down mode by the chip
enable pin (CE pin) to reduce current consumption. An
internal 100 nA pullup current source tied the CE pin to
OUT pin by default i.e. user can float the pin CE for
permanent “ON”. When voltage at pin CE is equal to or
greater than 0.9 V, the chip will be enabled, which means the
device is in normal operation. When voltage at pin CE is less
than 0.3 V, the chip is disabled, which means IC is shutdown.
During shutdown, the IC supply current reduces to 0.3 mA
and LX pin enters high impedance state. However, the input
remains connected to the output through the inductor and the
Schottky diode, keeping the output voltage to one diode
forward voltage drop below the input voltage.
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